Electroslag welding (“ESW”) and Narrow Gap Electroslag welding (“ESW-NG”) are enjoying resurgence in popularity in high-rise and medium-rise steel buildings now that demonstrable ESW and ESW-NG systems and methods for stiffener welding, column splicing, beam-to-column connections, and doubler plate moment connections have been developed and used. Principal considerations for use of ESW and ESW-NG are the significant cost savings in labor and materials.
Direct current (“DC”) welding power supplies have often been used as the standard power source for ESW and ESW-NG welding applications. The original ESW and ESW-NG welding applications utilized constant current (drooping Amp/Volt output curve) power sources without much success. It was not long before constant potential (also referred to as “constant voltage”) DC welding power sources were introduced to the process. Constant voltage power supplies set the voltage to be constant during the welding operation constant voltage welding power supplies use variable wire feed speed wire-feeders to adjust the load (or current draw) on the power supply. Constant voltage power supplies have a flat Amp/Volt output curve. When these power supplies are used for ESW and ESW-NG welding, they are generally connected electrode positive (“DCEP”—also referred to as DC reverse polarity). DC power supplies with electrode negative (“DCEN”—also referred to as DC straight polarity) are rarely used for the ESW and ESW-NG process because of poor arc starting and poor welding characteristics.
Ferrous materials, most commonly steel, can be easily magnetized by passing a DC welding current through the welding joint. Because of the magnetic properties of ferrous materials, high current DC welding is often not satisfactory. A magnetic field is generated by the DC arc because the current only flows in one direction. The magnetic field creates an electromagnet out of the material being welded. The electrical field surrounding the arc and the fields associated with the ground currents react with each other in an unpredictable manner. In submerged arc welding (SAW) this is often referred to as “arc-wander” or, “arc-blow”. When used with the ESW and ESW-NG welding process, this effect is most objectionable where erratic pull on the molten weld puddle, or the consumable guide tube, can disturb the proper formation and placement of the weld puddle. Welding with a DC power supply can easily create this strong magnetic deflection because DC current only flows in one direction. This unidirectional current flow magnetizes the parent material and results in pulling the molten puddle or the consumable guide tube to one side or the other, depending on the direction of the magnetic field.
To eliminate the problems created by welding in a strong magnet field, users generally revert to alternating current (“AC”) power. However the constant current AC sine wave AC does not perform well with the ESW and ESW-NG welding process. A constant current AC transformer has a very steep slope, see e.g. FIG. 5. Since it is constant current, the operator can only set the current on the power supply at a constant level during the welding operation, and not the voltage. This makes control of penetration into the parent material extremely difficult, if not impossible. A constant current type power source often causes arc starting to “stumble” and extinguish, thus failing to re-ignite. This poor starting condition exists because the AC sine wave exhibits a slow “crossover” where the voltage is very low (or zero) at the rate of 120 times per second. It is during the set-times that the arc goes out, and then the wire strikes the puddle again to begin re-ignition. This creates a very unstable arc condition and it causes many “arc start” failures when trying to get the ESW and ESW-NG weld puddle established.
Constant current AC power supplies (often referred to as “drooper” power supplies) are typically 80 volts open circuit and designed with transformers of high leakage construction. They are basically constant current power sources with the welding voltage established by the load, not by the power source. The load on the power supply is created by the diameter, the speed, and the number of welding wires. To use such a current source with the submerged arc process welding requires employment of a “voltage-following” wire feeder. The ESW and ESW-NG process only uses a welding arc until the molten electroslag welding puddle is established, then the arc is extinguished. After the puddle is formed, the arc is extinguished and there no longer is any arc to control the arc voltage. Welding stability with this type of AC welding power supply is difficult to obtain and hold. Current, voltage, and wire feed rate all interact with each other in an unpredictable manner, and the system is thus not stable and self-regulating. Moreover, starting current is very limited with constant current (drooper) AC power sources resulting in arc starts of marginal quality.
Square wave arc welding power supplies are known in the prior art. One example is shown in U.S. Pat. No. 3,364,334 to Sato. The Sato patent shows a diode bridge added in series with the AC line, and an inductor is connected across the DC terminals of the bridge. In this arrangement, the bridge is not the control element, and the welding current control must be provided on the AC line. In addition, the diodes of the bridge form a freewheeling path at all times.
In U.S. Pat. No. 4,038,515 to Risberg, the diodes and the bridge are replaced by thyristors. The actual welding current is compared with the desired weld current setting, and a firing control circuit responds to the resulting error signal to fire the Thyristor Bridge with proper phasing to supply a constant current to the load. This system cannot be used with a constant speed wire feeder. Therefore, the Risberg system could not be used in ESW or ESW-NG welding process unless it was tied to a variable speed wire feeder. The use of a variable speed voltage following wire feeder is not, however, a satisfactory solution because, as previously mentioned, cold starts are difficult, and any disturbance of the current or voltage would cause the voltage following wire feeder to react and interact with the arc in such a way as to become at least momentarily unstable.
U.S. Pat. No. 4,322,602 to Grist applies a square wave AC power source having constant potential power which can be matched with a constant speed wire feeder in deep groove submerged arc welding applications. The Grist square wave AC power source has a very rapid crossover transition at polarity reversal, and thus the arc does not extinguish each half cycle. As a result, the wire does not need to advance, strike the work-piece, ignite a new arc, and burn back 120 times per second. Grist does not have suitable properties for ESW and ESW-NG welding processes since it is tied to a constant feed wire feeder.
What is needed is a VB/CP/AC Square Wave power source, controlled in a constant voltage mode, FIG. 6, for the ESW and ESW-NG welding processes. This VB/CP/AC Square Wave power supply can be matched with 1) multiple guide tube configurations, 2) multi-pass applications, and/or 3) controllable variable speed wire electrode feeder assemblies, thus gaining the various advantages which have become familiar in DC automatic welding applications, namely, controlling the base metal dilution, the total heat input into the parent material and the reduction or elimination of any DC magnetic field.